Method of making carbon nanotube composite materials

ABSTRACT

The present invention is a method of making a composite polymeric material by dissolving a vinyl thermoplastic polymer, un-functionalized carbon nanotubes and hydroxylated carbon nanotubes and optionally additives in a solvent to make a solution and removing at least a portion of the solvent after casting onto a substrate to make thin films. The material has enhanced conductivity properties due to the blending of the un-functionalized and hydroxylated carbon nanotubes.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application for patent is related to U.S. patent applicationSer. No. 13/223,672 filed Sep. 1, 2011 and entitled “Carbon NanotubeComposite Materials,” which is assigned to the same assignee as that ofthe present invention and fully incorporated herein by reference. Thepresent application claims priority to prior provisional U.S. PatentApplication Ser. No. 61/411,308 originally filed Nov. 8, 2010 entitled“NANOTUBE SURFACE FUNCTIONALIZATION EFFECTS IN BLENDED MULTI-WALLEDCARBON NANOTUBE/PVDF COMPOSITES” from which benefit is claimed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofcontract DE-AC04-94AL85000 awarded by the U.S. Department of Energy toSandia Corporation.

BACKGROUND OF THE INVENTION

The present invention relates to a method of making carbon nanotubecomposite materials and, more particularly, to a method of makingblended composite polymeric material with both un-functionalized andhydroxylated carbon nanotubes.

Carbon nanotube (CNT) filled polymer nanocomposites are promisingmaterials for a variety of applications, with enhanced mechanical,thermal and electronic properties imparted to the polymer matrix bymixing even minute amounts (<0.1 wt %) of CNTs. These polymer compositematerials containing carbon-based fillers (carbon nanotubes) can beuseful because of their potential for high conductivity, low weight andease of processing. Of particular interest is the formation of CNTnetworks in a polymer film to create electrically conductive pathways.Useful would be an efficient method for imparting the high electricalconductivity of individual CNTs to a composite material. For a networkof CNTs to form an electrically conductive path requires a minimumoverlap of tubes, which is otherwise known as the percolation threshold.Van der Waals interactions between CNTs leads to aggregation andbundling into large clusters, which is a fundamental challenge toachieving maximum dispersion and conductivity for a given amount ofCNTs. To compensate for this aggregation and achieve a conductivepathway through a filled polymer matrix, the fraction of CNTs isgenerally increased beyond the minimum fill required for a theoreticalpercolation network. One method to increase the dispersion of CNTs invarious matrix materials is through pre-treatment functionalizationmethods and mixing procedures to obtain enhanced properties at lowerfill percentages. Generally, the increased dispersion fromfunctionalization of CNT outer walls comes at the expense ofcarbon-carbon bond cleavage and reduction in the π-bonding network,which leads to a loss of electrical conductivity. Another CNT dispersionenhancement strategy is to utilize small molecule and oligomericadditives that have high affinity for the CNT surface and can wraparound tubes to reduce aggregation via non-covalent functionalization.This strategy has proven highly useful at maintaining the desired chargetransport properties of CNTs while enhancing dispersion to formpercolated networks at lower fill percentages.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which form a part of the specification,illustrate one or more embodiments of the present invention and,together with the description, serve to explain the principles of theinvention. The drawings are only for the purpose of illustrating one ormore preferred embodiments of the invention and are not to be construedas limiting the invention. In the drawings:

FIG. 1A shows FT-IR spectra of PVDF; FIG. 1B shows FT-IR spectra of PVDFwith 1 wt % multi-wall carbon nanotubes (MWCNT); and FIG. 1C shows FT-IRspectra of PVDF with 1 wt % hydroxylated multi-wall carbon nanotubes(HO-MWCNT).

FIG. 2 shows Raman spectra of 1 wt % HO-MWCNT in PVDF and 1 wt % MWCNTin PVDF.

FIG. 3 shows the change in conductivity with increasing concentration ofMWCNT at three concentrations of HO-MWCNT.

FIG. 4 shows the change in conductivity with increasing concentration ofHO-MWCNT at varying concentrations of un-functionalized MWCNT.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention is a method of making acomposite material consisting essentially of a vinyl thermoplasticpolymer, un-functionalized carbon nanotubes and hydroxylated carbonnanotubes dissolved in a solvent (O'Bryan, G., Yang, E. L., Zifer, T.,Wally, K., Skinner, J. L, and Vance, A. L., Journal of Applied PolymerScience, 2011, 120, 1379-1384, herein incorporated by reference). Thematerial is also referred to as a nanocomposite material as carbonnanotubes are used to fill the polymeric material. The formed solutioncan be cast onto a substrate to make thin films with enhancedconductivity properties.

The composite material is a binary mixed-fill system employinghydroxyl-functionalized (hydroxylated) carbon nanotubes (HO-CNTs) withun-functionalized carbon nanotubes (CNTs) in a vinyl thermoplasticpolymer matrix. The vinyl thermoplastic polymer matrix includes but isnot limited to poly(vinylidene fluoride) (PVDF), polypropylene (PP)compounds, polyethylene (PE) compounds, polystyrene (PS) compounds,polyacrylate (PA) compounds, polyvinylchloride (PVC) compounds,polyvinyl acetate (PVA) compounds, and copolymers thereof. The polymeris dissolved in a solvent capable of dissolving the polymer and CNTs,including but not limited to dimethylformamide (DMF), dimethylacetamine,and methylpyrrolidone solvents. These solvents can optionally be mixedwith up to approximately 70 wt % of another solvent including acetone,chloroform, chlorobenzenes, toluene, xylenes, tetrahydrofuran,2-methyltetrahydrofuran, ethyl acetate, and cyclopentyl methyl ether.The concentration of the polymer can be up to the solubility limit ofthe polymer in the solvent system chosen.

For both the un-functionalized and hydroxyl-functionalized CNTs, eithersingle wall carbon nanotubes (SWCNT) or multi-wall carbon nanotubes(MWCNT) can be used. Single-walled nanotubes consist of a singlegraphite sheet wrapped into a cylindrical tube. Multi-walled nanotubesare an array of such single-walled nanotubes that are concentricallynested. The single-walled CNTs can be either metallic or semiconducting,depending on the sheet direction about which the graphite sheet isrolled to form a nanotube cylinder. In general, the electronicproperties of multi-walled CNTs are similar to those of single-walledCNTs. The concentration of the un-functionalized can vary from greaterthan 0 wt % up to approximately 30 wt % (calculated as weight percent ofthe composite solid material without including the solvent) and theconcentration of the hydroxyl-functionalized CNTs can vary from greaterthan 0 wt % up to approximately 40 wt % (calculated as weight percent ofthe composite solid material without including the solvent).

To aid in dispersion other additives can optionally be added to thesolution. Typical additives include, but are not limited to,surfactants, including a fluorosurfactant such as Zonyl® UR (Dupont) andnon-ionic surfactants such as Triton® X-100 (Dow), and compounds such aspolystyrene sulfonate and polyethylene glycol. Typical concentrationsrange up to 10 wt %.

In one embodiment, PVDF was chosen as the matrix material due to itsexcellent coating properties, such as high durability, solventresistance, and low creep, while maintaining solution processability forcasting thin films of composite materials. Herein, as shown insubsequent examples, blending hydroxylated MWCNTs with untreated MWCNTsin a PVDF nanocomposite leads to enhanced electrical characteristics.The enhanced electrical properties are observed at lower fills ofuntreated MWCNT compared to a non-blended (strictly untreated MWCNT/PVDFcomposites) system. The benefit of blending functionalized HO-MWCNT intoMWCNT/PVDF composites is observed from the electrical conductivitymeasurements of cast composite films. A power law fit of theexperimental data reveals a decrease in the percolation threshold withan increase in the saturation conductivity. The percolation thresholdwas decreased from 0.49 wt % to 0.25 wt %, with an increase inconductivity at the percolation threshold with only half theconcentration of MWCNT (ternary composite containing 10 wt % HO-MWCNT inPVDF) compared to the binary composite without functionalized MWCNT. Theimproved electrical properties point towards an improved dispersion ofMWCNT at lower fill percentages leading to a faster approach to thesaturation conductivity observed in this system.

Carbon nanotubes can be blended into a polymer matrix via melt,extrusion, or solution mixing. In one embodiment, the invention is amethod for preparing the solution comprising the vinyl thermoplasticpolymer with the mix of both the un-functionalized CNTs and hydroxylatedCNTs. In one specific embodiment to produce thin films of the polymernanocomposite materials, a solution-based process is used wherein aspin, dip, brush, spray coating or drop casting method allows formationof thin films on a substrate. To prepare samples, the vinylthermoplastic polymer, such as PVDF, is first dissolved into a solvent,such as a dimethylformamide (DMF) solvent or dimethylacetamine solvent.In another embodiment, the solvent N-methylpyrrolidone (NMP) was used,which creates excellent dispersions of CNT/polymer composites but ismuch more difficult to remove by evaporation after casting. Optionally,heating and agitation can be utilized to aid in the dissolution. Thepolymer can be added at any concentration up to the limit of solubilityof the polymer in the solvent. Optionally, additives, such as thesurfactants or other additive compounds noted previously, can be mixedinto the solvent. Both un-functionalized CNTs (at concentrations up toapproximately 30 wt %) and hydroxylated CNTs (at concentrations up toapproximately 40 wt %) are blended into the polymer solution withultrasonication optionally used to aid in dispersing the CNTs into thepolymer. Poor dispersion can cause the presence of large entanglementsof nanotubes that reduces the value of electrical conductivity and alsoyields inferior mechanical properties. Sonication times were kept brief(approximately 10 min) to minimize tube degradation. Solutions preparedin this manner were stable for about several months. The polymer matrix,hydroxylated CNTs, un-functionalized CNTs and the optional additives canbe mixed in the solvent in any order desired.

To produce thin films (generally of less than approximately 50 μm toabout 100 μm), after ultrasonication, the solutions were drop-castedonto glass plates at least a portion of the solvent removed; the solventremoval can be performed at any desired environmental conditions,including both elevated temperature or non-ambient pressure, such as ina pre-heated vacuum oven (70° C.) for removal of solvent. Thicker filmscan be optionally made by variation of the solution viscosity or byrepeating the casting procedure of the solution onto the coatedsubstrate. After drying, sample strips were submersed in liquid nitrogenand immediately fractured to reveal accurate cross-sectionalrepresentations. The composite films were then imaged on across-sectional puck at an accelerating voltage of 15 kV.

In one embodiment, solutions of 1 wt % MWCNT and 1 wt % HO-MWCNT in PVDFwere examined by FT-IR spectroscopy to determine interaction effectswith the matrix. Un-doped PVDF, FIG. 1A, has characteristic peaks at445, 512, 840, and 1175 cm⁻¹ corresponding to the β crystalline phase ofPVDF. No peaks were detected that correspond to the α crystalline PVDFphase, consistent with reports of PVDF crystal phases evolving from DMFsolutions. PVDF films doped with 1 wt % MWCNT and 1 wt % HO-MWCNTproduced virtually identical spectra. Enhancement of the β crystallinephase of PVDF was observed in both nanocomposite samples with peaks at512 cm⁻¹ and 840 cm⁻¹ increasing most dramatically. In the case of PVDFwith 1 wt % HO-MWCNT (cf. FIG. 1B) no broad signal was detected betweenabout 3200 cm⁻¹ to about 3600 cm⁻¹, a typical absorption for mosthydrogen bonded hydroxyl groups. Casting hydroxy functionalized MWCNTwithout PVDF and examining the material by FT-IR also did not yield theexpected H-O signal. Even still, the dispersion of HO-MWCNT in DMF andPVDF solutions remained much higher than un-functionalized MWCNT.

Solutions of 1 wt % MWCNT and 1 wt % HO-MWCNT in PVDF were also examinedby Raman spectroscopy. Two peaks were observed at 1350 cm⁻¹ and 1590cm⁻¹. The first peak, at 1350 cm⁻¹, corresponds to the disorder mode orD band and the second peak, at 1590 cm⁻¹, corresponds to the graphitictangential mode or G band. In SWCNTs, an increase in the D to G bandintensity is usually used to signify functionalization of the tubes. Thechange is attributed to defect sites forming during the transformationof carbon atoms from sp² to sp³ hybridization. In the Raman spectracollected in our samples however, the ratio of D to G band intensitiesappears to decrease for the sample consisting of HO-MWCNTs as comparedto the sample with un-functionalized MWCNTs. The explanation is that thesp² to sp³ hybridization only occurs in the outermost nanotubestherefore the change in intensity of the D band relative to the G bandbecomes less significant. Furthermore, the D band can actually be seento decrease in functionalized samples due to removal of defect ladenamorphous carbon during the functionalization process.

Differential scanning calorimetry (DSC) was performed on composites ofPVDF to determine crystallinity and observe thermal behavior inpolymer-MWCNT composites. No decrease in the polymer melt temperature(T_(m)) was observed in either nanocomposite, but a change in thepolymer crystallinity reveals an association difference between thefunctionalized and non-functionalized nanotubes and the matrix material.The decreased crystallinity of the non-functionalized nanotube PVDFcomposite is most likely the result of increased crystallite nucleation,consistent with the decrease in crystallinity observed in other PVDF CNTcomposite systems. However, the HO-MWCNT PVDF nanocomposite hasvirtually the same crystallinity as the pure PVDF material indicating afavorable CNT polymer interaction and therefore a decrease in the numberof nucleation sites.

In another embodiment, solution samples consisting of 2 wt % MWCNT, amixture of 1 wt % MWCNT and 1 wt % HO-MWCNT, and 2 wt % HO-MWCNT wereprepared for scanning electron microscope (SEM) analysis. Afterultrasonication, the solutions were immediately drop casted onto glassplates and placed in a pre-heated vacuum oven (70 C) for removal ofsolvent. After drying, sample strips were submersed in liquid nitrogenand immediately fractured to reveal accurate cross-sectionalrepresentations. The composite films were then imaged on across-sectional puck at an accelerating voltage of 15 kV.

The most noticeable difference between the three samples was filmthickness, namely 44 μm, 20 μm, and 12 μm for the 2 wt % MWCNT sample,mix of 1 wt % MWCNT and 1 wt % HO-MWCNT, and 2 wt % HO-MWCNTrespectively. The final film thickness was repeatable for a givenmix-fill concentration, and is dependent upon solution viscosity (on theorder of 100 CP for starting PVDF solutions) which changes with fillratio, polymer molecular weight and concentration. Since the total MWCNTweight percent of the samples was the same, the difference in thicknesswas attributed to the makeup and surface functionalization of the tubeswhich provides an increased interaction with solvent and matrixmaterials leading to enhanced dispersion. Further experiments revealedthat film thickness was largely dependent on weight percentage ofHO-MWCNT. Magnified images of the cross-sectional plane of thecomposites reveal a dense forest of tubes within the polymer.Qualitative comparison of images suggests an apparent decrease in MWCNTdensity corresponding to direct decrease in the MWCNT content. No otherapparent differences between the three compositions could be detected atthis level.

Initial studies to increase the dispersion of MWCNTs in PVDF revealedthat blending mix-fill systems comprised of functionalized andun-functionalized MWCNTs resulted in a slight decrease in sheetresistance. A series of electrical percolation experiments wereconducted to find an optimum blend of oxidized and untreated MWCNT inPVDF. Films of nano-composites were prepared via drop casting fromsolution. Circular discs of composite material with a diameter of 4 cm

Clearly, the ability of MWCNT to form an electrically conductive networkis aided by the presence of hydroxylated CNTs in a mix-fill system withthe un-functionalized CNTs. It should be noted that PVDF compositesconsisting of only HO-MWCNTs exhibited strictly insulating electricalproperties; therefore, the measured conductivities of the mix-fillmaterials were negligibly affected by current flow through theadditional HO-MWCNTs. It is possible that the dispersion of MWCNTs in amix-fill system is much greater and aggregation of the tubes during filmdrying is most likely decreased. Electrical percolation in polymernanocomposites with CNT's as the filler material has been examinedextensively, and yet improving dispersion and electron transport throughnanotube contact invariably remains a challenge in homogeneous polymernanocomposites (as opposed to segregated networks).

The behavior of the mixed-fill composites was further examined byfitting the electrical conductivity data to a power law dependence andextracting the characteristic parameters. Traditional percolation theorystates that conductivity of the composites can be described by theequation: σ(x)=a(x−b)^(n), where σ is electrical conductivity, a is thesaturation conductivity, b is the percolation threshold, and n is thecritical exponent. The saturation conductivity of the system shouldtheoretically approach the conductivity of the carbon nanotubes. Thepercolation threshold is the point at which the system becomesconductive and has been found to be a function of filler aspect ratio.And the critical exponent is a function of the overall system, includingfabrication of composites and resulting aggregation characteristics. Thecurve fits were based on an unconstrained nonlinear minimization of thesum of squared residuals with respect to the three parameters. Theextracted parameters are shown in Table 1 below.

TABLE 1 Extracted parameters for power law curve fit to conductivitydata 0 wt % 5 wt % 10 wt % Parameter HO-MWCNT HO-MWCNT HO-MWCNT a 0.110.30 0.92 b 0.49 0.25 0.25 n 1.58 1.13 0.30

The percolation threshold with no oxidized MWCNTs is seen to occur at0.49 wt % MWCNT. This threshold dramatically decreased to 0.25 wt %MWCNT when the same MWCNT were mixed into solutions containing 5 wt %HO-MWCNT. The amount of MWCNTs needed to form a conductive networkthrough the matrix in this case was nearly half as much. Not only wasthe percolation lower but the overall electrical conductivity of thesamples was increased as well. Increasing the concentration of HO-MWCNTfrom 5 wt % to 10 wt % resulted in a percolation threshold that wasapproximately the same as the threshold observed in 5 wt % HO-MWCNTsystem. However, comparing the lowest fractions of MWCNT (0.25 wt %),the electrical conductivity in the 10 wt % HO-MWCNT was three orders ofmagnitude higher than in the 5 wt % HO-MWCNT. Finally, there was asignificant change in the critical exponent with the addition offunctionalized MWCNTs from around 3 to 1. Since the fabrication methodof all composites was the same, this suggests that aggregation ofnanotubes was decreased in the systems with functionalized tubes.

FIG. 4 illustrates the change in conductivity with increasingconcentration of HO-MWCNT at varying concentrations of un-functionalizedMWCNTs. For the composites containing 0.25 wt % and 0.4 wt % with noHO-MWCNTs, there is no measurable conductivity and is plotted at 10⁻¹²S/cm to reflect the limitations of the measurement device. As the amountof MWCNTs is increased, the benefits of the addition of functionalizedMWCNTs decreases. At around 4.0 wt % MWCNT, the effect appearsnegligible. At this loading, it is likely that there is sufficientamount of MWCNTs present to make aggregation a nonissue. Furthermore,the boost in conductivity from 0 wt % to 5 wt % HO-MWCNT is moresignificant than from 5 wt % to 10 wt % HO-MWCNT suggesting there issome saturation limit to the benefits of incorporating HO-MWCNT.

EXAMPLES Materials and Sample Preparation

Multi-wall carbon nanotubes (MWCNT) were purchased in powder form fromCheap Tubes, Inc., with a purity of >95%, a tube diameter range of about8 nm to about 15 nm and a length of about 10 μm to about 50 μm. TheMWCNT were purified by washing with concentrated hydrochloride acidfollowed by a rinse with deionized water and a subsequent drying step inan oven at 120° C. for 18 hours. Hydroxy functionalized MWCNT (HO-MWCNT)purchased from Sun Innovations (Sun_(nano)) in powder form, with apurity of 95%, a tube diameter range of about 30 nm to about 50 nm and alength of about 10 μm to about 20 μm were used as received. Multi-wallCNTs were chosen for use in the examples herein because of the ease ofavailability; single wall CNTs could optionally been used. PVDF inpellet form, purchased from Sigma-Aldrich, had a weight average molarmass (M_(w)) of 180 kDa and a number average molar mass (M_(n)) Of 71kDa.

Instrumentation.

Ultrasonication was performed with a Sonics 750 W Vibra Cellultrasonicating processor with a ¼ in. microtip attachment operating at35% amplitude. Microscopy was performed on a Hitachi Scanning ElectronMicroscope S-4500 with an accelerating voltage of 15 kV. Electricalresistivity measurements were performed with a Jandel RM3-AR I-Vsource-meter using a Guardian SRM-232-10 four-point sheet resistanceprobe with a 62.5 mil spacing. A 100Ω resistor supplied by themanufacturer was used to calibrate the instrument. Fourier-transforminfrared (FT-IR) spectroscopy was performed on a Varian Scimitar Series800; solutions of composite material were cast onto transparent KBrplates for measurement. Raman spectroscopy was performed using the 532nm line of a Nd:YAG laser as the excitation source. The total powerincident on the substrate surface was approximately 2 mW and the laserspot size was approximately 650 nm in diameter. Spectra were collectedfor an interval of 60 seconds. Differential scanning calorimetry (DSC)was performed on a Mettler Toledo DSC822^(e) heating from roomtemperature to 200° C. at a rate of 2° per min in a 40 μL aluminum panwith a blank pan for reference.

Example 1 Preparation of Carbon Nanotube Composite Materials in a DMFSolvent

Solutions of PVDF in dimethylformamide (DMF) were prepared by heatingpre-weighed pellets of PVDF in DMF in a water bath at 80° C. withmechanical stirring until the pellets completely dissolved. Nanotubeswere weighed on an analytical balance, placed directly in PVDF solution,and vortexed to premix the solution. In various embodiments, MWCNTs atconcentrations varying from approximately 0.25 wt % up to 4 wt % wereused in the formulations and HO-MWCNTs at concentrations varying fromgreater than 0 wt % up to approximately 10 wt % were used in theformulations. Optionally, the vinyl thermoplastic polymer and CNTs canbe mixed in any order in the solvent.

Example 2 Preparation of Carbon Nanotube Composite Materials in aMethylpyrrolidone Solvent

Solutions of PVDF in N methyl-2-pyrrolidone were prepared by heatingpre-weighed pellets of PVDF in DMF in a water bath at 80° C. withmechanical stirring until the pellets completely dissolved. Nanotubeswere weighed on an analytical balance, placed directly in PVDF solution,and vortexed to premix the solution. Optionally, the vinyl thermoplasticpolymer and CNTs can be mixed in any order in the solvent.

Example 3 Preparation of Thin Film Composite Materials

Solutions of PVDF in dimethylformamide were prepared by heatingpre-weighed pellets in DMF in a water bath at 80° C. with mechanicalstirring until the pellets completely dissolved and adding desiredquantities of un-functionalized CNTs and hydroxylated CNTs. In oneembodiment, a solution was formulated with a mixture of 1 wt % MWCNT and1 wt % HO-MWCNT; one control solution containing 2 wt % MWCNT andanother control solution containing 2 wt % HO-MWCNT were also prepared.Other various embodiments used MWCNT concentrations from 0.25 wt % up to4 wt % and HO-MWCNT concentrations from greater than 0 wt % up to 10 wt%. After ultrasonication, the solutions were immediately drop castedonto glass plates and placed in a pre-heated vacuum oven (70 C) forremoval of solvent. The solution was chilled in an ice bath, and anultrasonicating probe tip was placed in the solution for sonication at apower output of 14 W for 10 minutes. The sonicated solution was dropcasted on a substrate, such as a glass plate, and dried in a vacuum ovenat 70° C. until the solvent was completely removed and the compositesurface was no longer glossy (about 10 to about 15 minutes). Aftercooling to room temperature, samples for resistivity measurements werecut on the glass plate into 4-cm-diameter circles and then peeled off.SEM analyses gave thin film thickness of 44 μm, 20 μm, and 12 μm for the2 wt % MWCNT sample, mix of 1 wt % MWCNT and 1 wt % HO-MWCNT, and 2 wt %HO-MWCNT respectively.

Although the invention has been described with reference to one or moreparticular embodiments, this description is not meant to be construed ina limiting sense. Various modifications of the disclosed embodiments aswell as alternative embodiments of the invention will become apparent topersons skilled in the art. It is therefore contemplated that theappended claims will cover any such modification or embodiments thatfall within the scope of the invention. The entire disclosures of allreferences, applications, patents and publications cited above arehereby incorporated by reference.

We claim:
 1. A method of making a composite material, comprising:dissolving a vinyl thermoplastic polymer, un-functionalized carbonnanotubes and hydroxylated carbon nanotubes in a solvent to make asolution; and removing at least a portion of the solvent.
 2. The methodof claim 1, wherein said solution is cast onto a surface prior to thestep of removing at least a portion of the solvent to form a film. 3.The method of claim 1, wherein said vinyl thermoplastic polymer isselected from the group consisting of poly(vinylidene fluoride), apolypropylene compound, a polyethylene compound, a polystyrene compound,a polyacrylate compound, a polyvinylchloride compound, a polyvinylacetate compound, and copolymers thereof.
 4. The method of claim 1,wherein said un-functionalized carbon nanotubes are selected from thegroup consisting of un-functionalized single-wall nanotubes andun-functionalized multi-wall nanotubes.
 5. The method of claim 1,wherein said hydroxylated carbon nanotubes is selected from the groupconsisting of hydroxylated single-wall nanotubes and hydroxylatednanotubes.
 6. The method of claim 1, wherein said un-functionalizedcarbon nanotubes comprise a concentration of greater than 0 wt % and upto approximately 30 wt %.
 7. The method of claim 1, wherein saidhydroxylated carbon nanotubes comprise a concentration of greater than 0wt % and up to approximately 40 wt %.
 8. The method of claim 1, whereinsaid solvent is selected from the group consisting of dimethylformamide,dimethylacetamine, and methylpyrrolidone solvents.
 9. The method ofclaim 8, wherein said solvent is mixed with up to approximately 70 wt %of a second solvent selected from the group consisting of acetone,chloroform, chlorobenzenes, toluene, xylene, tetrahydrofuran,2-methyltetrahydrofuran, ethyl acetate, and cyclopentyl methyl ether.10. The method of claim 2, wherein said film has a percolation thresholdof less than approximately 0.5%.
 11. The method of claim 2, wherein saidfilm has a conductivity of greater than 0.01 S/cm.
 12. The method ofclaim 2, wherein said solution is cast onto a surface by a methodselected from the group consisting of spin, dip, brush, spray coating ordrop casting.
 13. The method of claim 1, wherein said dissolving a vinylthermoplastic polymer, un-functionalized carbon nanotubes andhydroxylated carbon nanotubes in a solvent to make a solution is aid bya method selected from heating and agitation.
 14. The method of claim 1,wherein said un-functionalized carbon nanotubes have a concentration ofgreater than 0 wt % and less than approximately 4 wt %, and saidhydroxylated carbon nanotubes have a concentration of greater than 0 wt% and less than approximately 10 wt % to produce a film with aconductivity of greater than 10⁻⁵ S/cm.
 15. The method of claim 1,wherein an additive is added to the solvent.
 16. The method of claim 14,wherein said additive is selected from the group consisting of asurfactant, polystyrene sulfonate and polyethylene glycol.
 17. Themethod of claim 15, wherein said additive is added at a concentration ofless than 10 wt %.
 18. A method of making a composite material,comprising: dissolving a polymer, un-functionalized carbon nanotubes andhydroxylated carbon nanotubes in a solvent to make a solution; andremoving at least a portion of the solvent; wherein saidun-functionalized carbon nanotubes comprise a concentration of greaterthan 0 wt % and up to approximately 30 wt %; and wherein saidhydroxylated carbon nanotubes comprise a concentration of greater than 0wt % and up to approximately 40 wt %.
 19. The method of claim 18,wherein said un-functionalized carbon nanotubes are selected from thegroup consisting of un-functionalized single-wall nanotubes andun-functionalized multi-wall nanotubes.
 20. The method of claim 18,wherein said hydroxylated carbon nanotubes is selected from the groupconsisting of hydroxylated nanotubes and hydroxylated multi-wallnanotubes.